In general, a cross section of an outlet portion of a fuel nozzle of a solid-fuel burner has a shape close to a circle or a square, and a considerable distance is required for a flame ignited outside a fuel containing fluid jet to be propagated to a central portion of the fuel containing fluid jet in some situations. A distance along which an ignited flame in a jetting direction of the fuel containing fluid from the fuel nozzle is propagated to the central portion of the fuel containing fluid jet, i.e., an unfired distance increases as a diameter or an outer diameter portion of the fuel nozzle enlarges, and an unfired region expands. Promoting combustion in a reduction region near the burner is important to suppress generation of NOx in a combustion gas, but an increase in unfired region means a reduction in combustion time after ignition, and it can be a cause of insufficient suppression of NOx or a reduction in combustion efficiency.
In a boiler plant equipped with a plurality of solid-fuel burners as combustion apparatuses, although increasing a burner capacity is an effective technique for improvement of operability based on a reduction in cost and a decrease in the number of burners, there is a problem that the diameter or a length of the outer diameter portion of each fuel nozzle increases and an unfired region expands, which results in a cause of an increase in NOx and a reduction in combustion efficiency.
This problem is caused by a large distance from a fired region on a fuel containing fluid jet surface to the central portion of the fuel containing fluid jet.
WO2008-038426A1 (Patent Literature 1) discloses an invention that is the prior art according to the invention of the present applicant and that suppresses an unfired region while raising a burner capacity by a burner in which an outlet shape of a transverse cross section of a fuel nozzle is a rectangular shape having a long-diameter portion and a short-diameter portion, an elliptic shape, or a substantially elliptic shape and achieves prevention of an increase in NOx concentration in a combustion gas and a reduction in combustion efficiency of a fuel.
Further, WO2009-125566A1 discloses an opening shape of a burner similar to the above invention.
Furthermore, in a boiler plant, in a case where a fluid passes through a fluid passage for using vapor obtained by heating a fluid flowing through a plurality of heat transfer tubes by a high-temperature exhaust gas provided by solid-fuel burners in a boiler furnace and a complicated fluid passage for recycling the obtained vapor, obtaining a specific heat transfer quantity to a fluid in a heat transfer portion where each heat transfer tube is installed is important, and hence a temperature of the combustion gas and a flow rate of the fluid must be controlled with respect to each heat transfer portion. Therefore, there is an invention that can control a heat transfer quantity to a fluid in each heat transfer tube by changing a combusting position of a fuel in a furnace (WO2009-041081A1). In an example described in this invention, a gas jet nozzle outlet provided to a solid-fuel burner is divided into two, upper and lower pieces, and independently adjusting respective air flow rates enables changing a combusting position of the fuel vertically.
It is to be noted that the boiler that uses a solid fuel generally uses pulverized coal as the solid fuel, and hence such a boiler may be referred to as a pulverized coal burning boiler and a solid-fuel burner will be referred to as a pulverized coal burner hereinafter. At the time of starting the pulverized coal burning boiler, fans are activated, and air is supplied as a combustion gas to a plurality of pulverized coal burners installed in a boiler furnace and two-staged combustion air ports. Subsequently, a flame is formed with respect to a pilot torch of each burner, and a frame detector (which will be referred to as an FD hereinafter) detects this flame, and then a liquid fuel jetted from each ignition burner is ignited by the flame of each pilot torch to form the flame with respect to each ignition burner. After the FD detects the formation of the flame using the ignition burner, the fire of each pilot torch is extinguished, and a pilot torch gun is removed to the outside of the furnace to prevent burnout.
Then, a temperature of the furnace is increased by the ignition burner until a furnace outlet temperature reaches a set temperature, and a mill is activated to gradually switch to pulverized coal combustion. That is, in the pulverized coal burner, to ignite the pulverized coal, each ignition burner using a liquid fuel or the like is installed, and the pilot torch that ignites this ignition burner and the FD that detects flames are further installed.
As the pulverized coal burners, there is used a pulverized coal burner that has an ignition burner installed at the center, allows pulverized coal and primary air as a carrier gas to flow from the periphery of the ignition burner and ejects them into a furnace, and supplies combustion air from the periphery. In this case, to prevent a flow of the pulverized coal from being disturbed and avoid deposition of the pulverized coal or a flame stabilization failure in the burner, the pilot torch and the FD are installed in a combustion air supply unit at the periphery rather than a pulverized coal outlet portion.
If the FD or the pilot torch is installed in the combustion air supply unit like the prior art, there is a situation that an installation site may affect the flame detection using the FD or stable ignition and flame stabilization in the ignition burner using the pilot torch.
Further, there is also known that a flame detector is arranged in a tertiary air flow path in a pulverized coal burner (Japanese Unexamined Patent Application Publication No. Hei 4-268103).